Electrical instruments of the type herein contemplated (e.g. instruments for testing the electronic functions of automobile engines) are required to be shrouded in metal sheeting referred to as a card cage. The metal card cage constrains and protects the electrical field generated by the instrument's electronics, which is necessary to obtain accurate instrument readings. Whereas the card cage material is necessarily electrically conductive, for many applications of these instruments, the exposed outer shell of the housing is desirably made of a nonconductive material, e.g. plastic.
Heretofore the housing for such electrical instruments included fabricated metal sheeting (for the bottom, top and sides of the card cage). Numerous holes were provided in the sheeting to accommodate screws or press-fit posts used to attach the various electronic components to the card cage, and also to attach the card cage to the outer plastic shell. The assembly process involving the handling and attaching of many individual screws, was time consuming and expensive. The electronic components inside the housing have become less costly and thus the housing assembly has become an satisfactory disproportionate factor in the overall cost of the instrument.
In one attempt to reduce the assembly cost, the more efficient plastic fasteners were utilized in a modified housing wherein the plastic outer shell was first formed and the inner walls were then coated with a metal material. The process of metal coating is however, also very costly and the overall cost was not substantially reduced.